Process Control Loops.....................................................................................................................................46 Learning Objectives..................................................................................................................................46 Single Control Loops .......................................................................................................................................47 Feedback Control .....................................................................................................................................47 Examples Of Single Control Loops..................................................................................................................48 Pressure Control Loops............................................................................................................................49 Flow Control Loops..................................................................................................................................49 Level Control Loops .................................................................................................................................50 Temperature Control Loops .....................................................................................................................51

Pneumatic signalling is still common. The 3 corresponds to the lower range value (LRV) and the 15 corresponds to the upper range value (URV). Analog signal 3. and the transmitter is set to produce a 4 mA signal when the process temperature is at 95 °C and a 20 mA signal when the process temperature is at 105 °C. The transmitter will transmit a 12 mA signal when the temperature is at the 100 °C setpoint. An RTD temperature sensor and transmitter are installed in the process vessel.com
.PAControl.Components of Control Loops and ISA Symbology
Control Loop Equipment and Technology SIGNALS Activities
There are three kinds of signals that exist for the process industry to transmit the process variable measurement from the instrument to a centralized control system. Analog Signals The most common standard electrical signal is the 4–20 mA current signal. Other common standard electrical signals include the 1–5 V (volts) signal and the pulse output. For example. Digital signal Pneumatic Signals Pneumatic signals are signals produced by changing the air pressure in a signal pipe in proportion to the measured change in a process variable. This signal can be converted to a temperature reading or an input to a control device.
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Hydraulic signals Digital signals Analog signals Pneumatic signals Electro-magnetic signals
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Fundamentals of Control
www. The current signal is a kind of gauge in which 4 mA represents the lowest possible measurement. Identify the signal types that are used in the process control industry? Select all options that apply. 5. However. As the sensor’s resistance property changes in response to changes in temperature. such as a burner fuel valve. With this signal. or zero. Pneumatic signal 2. The common industry standard pneumatic signal range is 3–15 psig. a transmitter sends a small current through a set of wires. and 20 mA represents the highest possible measurement. the transmitter outputs a 4–20 mA signal that is proportionate to the temperature changes. imagine a process that must be maintained at 100 °C. the lower costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes has made pneumatic signal technology less attractive. since the advent of electronic instruments in the 1960s. 1.

Activities
16. The symbol displayed below denotes a PLC in a primary location. Is this statement true or false?

Control Room

Auxiliary

Field

Not accessible

PLCs

Two triangles with their apexes contacting each other (a “bow tie” shape) represent a valve in the piping. An actuator is always drawn above the valve (Figure 7.10).
Pneumatic valve Manual valve Electric valve